Understanding Diesel Exhaust Fluid: What to Do If the Light Goes On

Diesel exhaust fluid is a common component in the effort to clean up modern diesel tailpipe emissions. Known as DEF, or by various trade names such as AdBlue, the fluid is periodically injected into the exhaust where it works with a catalyst to reduce nitrogen oxide (NOx) emissions.

If you own a late-model diesel car or truck, there’s a good chance you’ll need to learn how to manage your vehicle’s DEF. Check out these helpful tips on what to do if a low-DEF warning light shows up on your dashboard.

Understand Your Options

Once you’ve purchased a car that requires diesel exhaust fluid, the first thing you should do is open up the owner’s manual and find out what will happen if the DEF light illuminates because you’re running low. Emissions regulations in the U.S. don’t allow diesel vehicles to operate in the absence of DEF if it’s required by their exhaust design. As a result, automakers have come up with a variety of strategies to deal with this situation. Generally, a warning light will come on when DEF needs to be added soon. If the light is ignored for too long, the vehicle will likely go into a reduced power mode. If the DEF level falls too low, the engine will not start until fluid is added. To avoid getting stranded, make sure you understand how your particular car or truck is designed to handle a low DEF condition.

Know Where to Get It

The best way to make sure you’re never stuck on the side of the road with an illuminated DEF warning light is to keep a supply on-hand, either at home or in the vehicle. DEF is non-flammable so it can be safely transported. The dealership you bought your vehicle from is likely to stock DEF, but you’ll save a lot of money by shopping around as all DEF is the same. If you’re traveling on the highway and need to top up, you can buy DEF at a NAPA AUTO PARTS store or have it topped up for you at a NAPA AutoCare location.

Fill Your Diesel Exhaust Tank

The final step for dealing with a DEF light is to know where the filler location is on your vehicle. Most pickups have it adjacent to the diesel fuel filler opening, but some cars and SUVs hide it under the hood, inside the trunk, beneath the spare tire, near the back seat or in a variety of other locations.

The exact details of your vehicle’s requirements will be listed in your owner’s manual. Save yourself some stress and make sure to crack it open and familiarize yourself with the DEF-filling process before you’re stuck doing it in a parking lot in the middle of the night.